The present invention relates to a buffer circuit and, in particular, to a buffer circuit which allows selection of operating states, that is, selection or non-selection, using an input signal, relative to a plurality of semiconductor devices, such as a semiconductor storage, according to a user's purpose.
Such a buffer circuit is used, for example, for making it possible to desirably select operating states of a plurality of semiconductor devices, such as a mask ROM, using an input signal according to a user's request. For example, a user may request that a semiconductor device be set to an operating state, that is, a selected state, when a low-level input signal is supplied. On the other hand, another user may request that a semiconductor device be set to an operating state when a high-level input signal is supplied. Further, another user may request that a semiconductor device be set to an operating state regardless of whether an input signal is at a low or high level.
For satisfying such requests, the buffer circuit is arranged so as to be capable of selecting one of three modes which correspond to the foregoing user's three kinds of requests, that is, the buffer circuit is arranged to be programmable. The buffer circuit of this type is disclosed, for example, in U. S. Pat. No. 4,612,459.
As will be described later in detail, this buffer circuit comprises a first stage circuit including an NOR circuit and a plurality of MOS transistors and a second stage circuit including an exclusive-OR circuit and a plurality of MOS transistors. Each of the foregoing MOS transistors changes from the enhancement type to the depletion type by ion implantation to change its conduction property. Specifically, the foregoing MOS transistors act as switching elements and are used for selecting one of the following three modes:
(1) In the first mode, a low-level output signal is produced as a selection signal when an input signal has a low level. This mode is called a low active mode.
(2) In the second mode, a low-level output signal is produced as a selection signal when an input signal has a high level. This mode is called a high active mode.
(3) In the third mode, a low-level output signal is produced as a selection signal irrespective of whether an input signal has a low or a high level. This mode is called a don't care active mode.
A user can designate one of the foregoing first to third modes. According to this designation, the ion implantation is selectively carried out relative to the foregoing plurality of MOS transistors during fabrication of the buffer circuit. For this reason, the foregoing buffer circuit is called a programmable buffer circuit.
However, the foregoing buffer circuit, particularly the second stage circuit thereof, requires the MOS transistors in large number since the exclusive-OR circuit is used. This results in a large chip area.